Heart valve regurgitation occurs when the heart leaflets do not completely close when the heart contracts. When the heart contracts, blood flows back through the improperly closed leaflets. For example, mitral valve regurgitation occurs when blood flows back through the mitral valve and into the left atrium when the ventricle contracts.
In some instances regurgitation occurs due to disease of the valve leaflets (e.g., primary, or “organic” regurgitation). Regurgitation can also be cause by dilatation of the left ventricle, which can lead to secondary dilatation of the mitral valve annulus. Dilation of the annulus spreads the mitral valve leaflets apart and creates poor tip cooptation and secondary leakage, or so-called “functional regurgitation.”
Currently, primary regurgitation is corrected by attempting to remodel the native leaflets, such as with clips, sutures, hooks, etc., to allow them to close completely when the heart contracts. When the disease is too far advanced, the entire valve needs to be replaced with a prosthesis, either mechanical or biologic. Examples include suture annuloplasty rings all the way to actual valve replacement with leaflets, wherein the suture rings are sutured to the mitral valve annulus. Annuloplasty rings, which are also sutured to the annulus, have also been used to attempt to remodel the annulus, bringing the native leaflets closer together to allow them to properly close.
Based on the success of catheter-based aortic valve replacement there is growing interest in evaluating similar technologies to replace the mitral valve non-invasively using similar types of replacement valves.
Unlike the aortic valve, however, the mitral valve annulus does not provide a good landmark for positioning a replacement valve. In patients needing a replacement aortic valve, the height and width of the aortic annulus are generally increased in the presence of degenerative disease associated with calcium formation. These changes in tissue make it easier to properly secure a replacement aortic valve in place due to the reduced cross-sectional area of the aortic annulus. The degenerative changes typically found in aortic valves are not, however, present in mitral valves experiencing regurgitation, and a mitral valve annulus is therefore generally thinner than the annulus of a diseased aortic valve. The thinner mitral valve annulus makes it relatively more difficult to properly seat a replacement cardiac valve in the native mitral valve annulus. The general anatomy of the mitral valve annulus also makes it more difficult to properly anchor a replacement heart valve in place. The mitral valve annulus provides for a smoother transition from the left atrium to the left ventricle than the transition that the aortic valve annulus provides from the aorta to the left ventricle. The aortic annulus is anatomically more pronounced, providing a larger “bump” to which a replacement aortic valve can more easily be secured in place.
In general, the aortic valve annulus is smaller than the mitral valve annulus. It has been estimated that the mitral valve annulus is about 2.4 cm to about 3.2 cm in diameter, while the aortic valve annulus has been estimated to be about 1.6 cm to about 2.5 cm in diameter.
The larger mitral valve annulus makes it difficult to securely implant current percutaneously delivered valves in the native mitral position. Current replacement aortic valves are limited in the amount of radial expansion they can undergo during deployment and implantation. To provide a replacement aortic valve that has an expanded configuration such that it can be securely anchored in a mitral valve annulus would require that the collapsed delivery profile of the replacement aortic valve be increased. Increasing the collapsed delivery profile, however, would make endovascular delivery more dangerous for the patient and more difficult to navigate the vasculature with a larger diameter delivery system.
Some attempts have been made to deliver and implant a one-piece replacement mitral valve, but it is difficult to provide a device that can be collapsed down to have a sufficiently small delivery profile and still be able to be expanded and secured in place within the mitral valve via a vascular access site.
A valve support structure or anchoring device is needed that can be positioned near or within the native mitral valve and that is adapted to secure a replacement cardiac valve in place.